Linking element for dynamically stabilizing a spinal fixing system and spinal fixing system comprising same

ABSTRACT

A linking element for a spinal fixing system is designed to link at least two implantable connecting assemblies. The linking element consists at least partly of a support made of polymeric material and a rod, bent or not, substantially coaxial with the support. The invention also concerns a spinal fixing system comprising at least two implantable connecting assemblies linked by at least one linking element of the invention.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to the field of spinal systems for linkingthe vertebrae.

(2) Prior Art

There are two types of spinal links: osteosynthesis links, and dynamiclinks.

Spinal osteosynthesis links have the aim of setting the linked vertebraein a specific configuration and of stabilizing them during bone fusionin order to permit fixed stabilization in the set position.

Dynamic links, by contrast, do not generate bone fusion but instead seekto reduce the stresses on the articular facets and on the intervertebraldisks, by permitting certain movements, while at the same timerealigning the vertebrae relative to one another, if necessary.

The present invention relates more particularly to a linking anddynamically stabilizing element for a spinal fixation system, designedto link at least two implantable connecting assemblies to one another,permitting certain possibilities of movement of one relative to theother.

Linking elements for dynamic stabilization are already known from theprior art.

European patent application EP669109 proposes a device for stabilizingadjacent thoracic vertebrae, comprising a band which is made of anelastic synthetic material and which has a round cross section, and atleast two pedicle screws. The band can be inserted through thetransverse hole in the head of the screws and fixed by a locking screwtransversely with respect to the hole, that is to say in the directionof the screw axis.

The device additionally comprises a bearing element mounted on the band.This bearing element forms a pressure-resistant body in order totransfer pressure forces between the two screw heads. The cross sectionof the band fits in suitable holes of the bearing element and of thescrew head in order to center the bearing element and the screw headrelative to one another.

In addition, the band can be pre-stressed between two adjacent pediclescrews by a band extension which protrudes outside the pedicle screws inorder to be able to mutually support the bearing element and the screwhead on a bearing surface which is common to them and is arranged aroundthe band.

A major disadvantage of this stabilizing device is that the band has tobe slid into the hole formed in the head of the pedicle screws, and alsothrough the bearing element(s). This first entails a difficulty, uponimplantation, in passing the band through these elements, as the pediclescrews are already fixed to the vertebrae. Introduction of the band isnot easy; but it means above all that the length of the bearing elementhas to be chosen before the band is put in place. It may well happenthat the effective distance between the screws after tensioning of theband is not exactly the distance desired, and in this case the surgeonhas no other choice than to dismantle the assembly made up of bearingelement and band in order to introduce a bearing element of a differentlength. In fact, this system does not afford any freedom in terms ofcompression or distraction between the screws after the elements haveall been put in place. Moreover, the viscoelastic material of thebearing element compresses when the band is tensioned, which furthercomplicates the choice of length of the bearing element since thischanges during tensioning of the band.

Another major disadvantage of the stabilizing device of the prior art isthat the linking element takes up a considerable volume: of the order of12.5 millimeters. In some circumstances, it is then difficult to avoidthe linking element coming into contact with bones and this contactcausing great pain.

Another major disadvantage of the stabilizing device of the prior art isthat the linking element does not effect any torsional return in orderto oppose pivoting movements of the vertebrae about the disks.

Another major disadvantage of the stabilizing device of the prior art isthat the linking element cannot be curved in order to adapt it to thenatural curvature of the lumbar spine.

Another major disadvantage of the device is the impossibility ofperforming surgical revision to extend the assembly, or of replacing thelinking element by a rigid rod in order to obtain osteosynthesis withoutcomplete removal of the material beforehand.

Another disadvantage of the device is the impossibility of combining itwith a conventional osteosynthesis system.

SUMMARY OF THE INVENTION

The present invention aims to overcome the disadvantages of the priorart by proposing a linking element for a spinal fixation system whichcan be inserted into an implantable connecting assembly, on the head ofthis assembly, in a conventional manner, such as a linking rod forosteosynthesis which has a small diameter, which provides torsionalreturn in order to incite a return to a position of equilibrium, whichcan be curved, which can be combined with an osteosynthesis system, andwhich can also be easily interchanged to perform osteosynthesis.

To do this, the present invention is of the type described above and itis distinguished, in its widest sense, by the fact that the linkingelement is composed, at least partly, of a support made of polymermaterial and of a rod, curved or not, substantially coaxial with saidsupport.

Said support preferably has a substantially tubular or cylindricalshape.

Advantageously, said linking element additionally comprises a helicalspring substantially coaxial with said support, said spring having turnswhich are at least partly embedded in said support.

Said rod preferably has an external diameter smaller than the internaldiameter of said turns.

To perform osteosynthesis, the linking element preferably comprises astraight or curved stiffening element.

This stiffening element is preferably composed of a sheet of materialwith a substantially U-shaped cross section.

The present invention also relates to a spinal fixation systemcomprising at least two implantable connecting assemblies linked by atleast one linking element according to the invention, said linkingelement being composed, at least partly, of a support made of polymermaterial and of a rod, curved or not, substantially coaxial with saidsupport.

To perform osteosynthesis, when the fixation system comprises astiffening element, this element is preferably fixed at least to the twoimplantable connecting assemblies.

To perform osteosynthesis, the fixation system may additionally compriseat least one rigid linking element.

Advantageously, the linking element according to the invention is easyto insert into the receiving means of an implantable connecting assemblyof known type. The placement of the linking element according to theinvention does not involve the use of an implant specifically designedfor this. The implant technique is therefore the same as for fusion.

Advantageously too, the structure of the linking element according tothe invention makes it possible to obtain the desired resistance tocompression and extension while requiring only a small volume. Inaddition, as the external diameter of the linking element according tothe invention is substantially the same as that of the linking elementsused for fusion, it is possible, within the same fixation system, toprovide a combination of transverse links for fusion and transverselinks for dynamic stabilization.

Advantageously too, the structure of the linking element according tothe invention also permits torsional return in order to force a returnto a defined configuration of equilibrium.

Advantageously too, the structure of the linking element according tothe invention allows it to be adapted to the implantation configurationby giving it the desired shape, and in particular by curving it inadvance.

Advantageously too, the structure of the linking element according tothe invention permits compression and distraction directly on thescrews.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood from the following description,given solely by way of example, of an embodiment of the invention, withreference being made to the attached figures in which:

FIG. 1 shows a front view of a first embodiment of the linking elementaccording to the invention, straight;

FIG. 2 shows a cross section along line AA in FIG. 1;

FIG. 3 shows a front view of a first embodiment of the linking elementaccording to the invention, curved;

FIG. 4 shows a front view of a second embodiment of the linking elementaccording to the invention, straight;

FIG. 5 shows a cross section along line AA in FIG. 4;

FIG. 6 shows a front view of a second embodiment of the linking elementaccording to the invention, curved;

FIG. 7 shows a front view of a third embodiment of the linking elementaccording to the invention, straight;

FIG. 8 shows a cross section along line AA in FIG. 7;

FIG. 9 shows a front view of a third embodiment of the linking elementaccording to the invention, curved;

FIG. 10 shows an exploded cross-sectional view of a first embodiment ofa connecting assembly for a linking element according to the invention;

FIG. 11 shows a front view of the connecting assembly from FIG. 10, whenmounted;

FIG. 12 shows a side view of the receiving means of the connectingassembly from FIG. 10;

FIG. 13 shows a front view of the receiving means of the connectingassembly from FIG. 10;

FIG. 14 shows a front view of a fixation system comprising twoconnecting assemblies according to the first embodiment, and a straightlinking element according to the invention;

FIG. 15 shows a front view of a spinal fixation system comprising fourconnecting assemblies according to the first embodiment, and a straightlinking element according to the invention, stiffened along a straightportion;

FIG. 16 shows a perspective view of FIG. 15;

FIG. 17 shows a front view of a fixation system comprising fourconnecting assemblies according to the first embodiment, and a curvedlinking element according to the invention, stiffened along a straightportion;

FIG. 18 shows a front view of a fixation system comprising fourconnecting assemblies according to the first embodiment, and a curvedlinking element according to the invention, stiffened along a curvedportion;

FIG. 19 shows a top view of a fixation system comprising twelveconnecting assemblies according to the first embodiment, four straightlinking elements according to the invention, two rigid linking elementsand four domino blocks, in order to link six vertebrae to one another;

FIG. 20 shows a cross section through a second embodiment of aconnecting assembly for a linking element according to the invention;

FIG. 21 shows a partial cross section through a third embodiment of aconnecting assembly for a linking element according to the invention;

FIG. 22 shows a perspective view of FIG. 21; and

FIG. 23 shows a front view of a fixation system comprising twoconnecting assemblies according to the second embodiment, two connectingassemblies according to the third embodiment, two curved linkingelements according to the invention, and a rigid linking element.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The linking element (101, 201, 201′) for a spinal fixation system (100,200) according to the invention is designed to link at least twoconnecting assemblies (102, 202) which are implantable in the spine withthe aid of bone-anchoring means.

An implantable connecting assembly (102) according to the firstembodiment of the invention is an assembly such as the one which isknown from international patent application WO 01/01873 and whosecooperation with the linking element according to the present inventionis described in greater detail below, with reference to FIGS. 10 through19.

An implantable connecting assembly (202) according to the secondembodiment of the invention is an assembly such as the one which isknown from international patent application WO 01/39677 and whosecooperation with the linking element according to the present inventionis described in greater detail below, with reference to FIGS. 20 through23.

The linking element (101, 201, 201′) is substantially tubular orcylindrical and has a longitudinal axis. Its external diameter is of theorder of 6 millimeters.

The linking element (101, 201, 201′) according to the invention iscomposed, at least partly, of:

on the one hand, a helical spring (150, 250) having an axis (155, 255)and turns (152, 252) and,

on the other hand, a support (160, 260) made of polymer material andhaving an axis (165, 265) substantially parallel with the axis of saidhelical spring (150, 250).

Said turns are at least partly embedded in at least one support (160,260) made of polymer material.

The helical spring (150, 250) can be made of metal, or of metal alloy,or of any type of biocompatible material permitting production of such aspring. It has turns (152, 252) with an external diameter (151, 251) andan internal diameter (153, 253). The diameter of the metal rod formingthe turns is of the order of 0.8 millimeter for a titanium-based alloymaterial.

The support (160, 260) can be made, for example, of elastic biomaterial(polycarbonate, polyvinyl, alcohol, etc.). It is of a substantiallytubular or cylindrical shape and has an external diameter (162, 262)and, if appropriate, an internal diameter (163, 263).

The external diameter (162, 262) of the support (160, 260) can begreater than the external diameter (151, 251) of the helical spring(150, 250), and the support (160, 260) can consequently completelyenclose the helical spring (150, 250); however this external diameter(162, 262) cannot be smaller than the internal diameter (153, 253) ofthe helical spring (150, 250). In other words, in cross section, theturns (152, 252) are always at least partly enclosed in polymer materialof the support (160, 260).

Although not illustrated here, it is also possible to imagine that theinternal diameter (163, 263) of the support (160, 260) is greater thanthe internal diameter (153, 253) of the helical spring (150, 250).

In the linking elements (101, 201, 201′) illustrated, the helical spring(150, 250) and the support (160, 260) have a substantially identicallength; however, it is possible to imagine that at one or both ends thespring is shorter than the support, or vice versa.

FIGS. 1 and 2 show a first embodiment of the linking element accordingto the invention, in which the support (160, 260) is cylindrical and hasan external diameter (162, 262) substantially identical to the externaldiameter (151, 251) of the turns (152, 252).

FIGS. 4 and 5 show a second embodiment of the linking element accordingto the invention, in which the support (160, 260) is cylindrical and hasan external diameter (162, 262) smaller than the external diameter (151,251) of the turns (152, 252), but without being smaller than theinternal diameter (153, 253) of the turns (152, 252).

FIGS. 7 and 8 show a third embodiment of the linking element accordingto the invention, in which the support (160, 260) is tubular and has anexternal diameter (162, 262) substantially identical to the externaldiameter (151, 251) of the turns (152, 252), and an internal diameter(163, 263) smaller than the internal diameter (153, 253) of the turns(152, 252). In addition, the linking element comprises a rod (170, 270)at its center. This rod (170, 270) has an axis (175, 275) substantiallycoaxial with the axis (155, 255) of said spring (150, 250).

This rod (170, 270) additionally has an external diameter (171, 271)smaller than the internal diameter (153, 253) of said turns (152, 252).

The rod (170, 270) can be made of metal, or of metal alloy, or of anytype of material permitting production of such a rod.

It should be noted that the rod (170, 270), the helical spring (150,250) and, consequently, the linking element (101, 201, 201′) can becurved, according to a desired curvature, as is illustrated in FIGS. 3,6 and 9, in order to permit adaptation of the linking element (101, 201,201′) to a particular configuration.

The structure of the linking element (101, 201, 201′) according to theinvention permits compression and distraction directly on the implantedconnecting assemblies, in order to permit dynamic stabilization.

The dynamic linking element according to the invention can also becombined, in the same fixation system, with rigid linking elements whoseaim is to permit osteosynthesis between the implants thus linked.

The linking element (101, 201, 201′) according to the invention can thusalso comprise a stiffening element (143, 143′), straight or curved, inorder to permit stiffening between at least two implantable connectingassemblies and thus permit osteosynthesis between the implants thuslinked (FIGS. 15 through 18).

This stiffening element (143, 143′) is composed, for example, of a sheetof material having a substantially U-shaped cross section, and thisshape can be introduced into the receiving means of the connectingassemblies and is fixed by the fixation means of the linking element onthe implantable connecting assembly.

In place of the stiffening element (143, 143′) associated with thelinking element (101, 201, 201′), it is possible to position a linkingelement (145, 245) formed by a straight or curved rigid rod (FIGS. 19and 23).

In the examples shown, the spring (150, 250) is made in one piece, andthe helical shape is identical along the whole length of the spring.

However, it is possible to imagine the spring being interrupted insidethe linking element, either by a rod, for example of parallel axis withthe rest of the linking element, or by the material from which thesupport (160, 260) is made.

It is also possible to imagine the pitch of the helical shape changinginstead of being constant.

It can additionally be imagined that the helical shape is not circular,but oblong, that the material forming the spring does not have acircular cross section, but an oblong one, or more simply that thecross-sectional shape of the support is not circular, but oblong, inorder to achieve greater strength along the principal length. Thereceiving means for the linking element on the implant would then beconsequently modified.

Linking Element According to the Invention Used with a ConnectingAssembly According to the First Embodiment

According to the first embodiment, shown in FIGS. 10 through 18, thefixation system comprises a linking element (101), an implant (102) witha complementary closure piece (103), and a locking screw (113), as isshown in FIG. 10.

The implant (102) has a fork-shaped head (105) with two lateral arms(106, 107) delimiting a space intended to receive the linking element(101).

The bottom (108) of the fork has, in cross section, the general form ofa horseshoe, with a concave curvature in the transverse planecorresponding to the plane in FIG. 10, and a convex curvature in thecomplementary plane.

The radius of concave curvature corresponds substantially to theexternal radius of the linking element (101). The latter thus makescontact along a semi-peripheral line. This contact by a line, ratherthan by an annular surface, permits a degree of freedom in pivoting andat the same time ensures more effective locking after tightening than inthe case of a simple point contact.

The closure piece (103) has a general U-shape with two arms (110, 111),and the bottom of the U has an internal thread (112) for receiving alocking screw (113).

The arms (110, 111) are at a distance permitting engagement on the head.At their lower end, the arms (110, 111) have arc-shaped shoulders (114,115) with an inclined upper surface (116, 117).

These arc-shaped shoulders (114, 115) cooperate with complementary guidemeans (120, 121) provided on the head (105). These guide means also havean arc-shaped and inclined contact surface (122, 123) and cooperate withthe complementary contact surfaces (114, 115) when the closure piece isengaged on the head (105). They then provide guiding so as to permitpivoting of the closure piece on a transverse axis (124) and they ensurelocking of the closure piece (103) on the head (105), and thus lockingof the linking element (101) after the screw (113) is tightened.

FIG. 11 is a side view showing that the linking element (1) has a degreeof freedom in terms of tilting about a transverse axis (124). Thispermits independence of the implant and makes it possible to positionthe implant by means of a hook (126) on the pedicle, and toindependently seek the best orientation of the linking element (1)without interference between these two constraints. The horseshoe shapeand the mobility of the closure piece makes it possible to adapt thelocking and to avoid unwanted rotation or displacement of the rod duringtightening of the screw (113).

The hook (126) delimits a U-shaped space (127) for connection to thelamina of a vertebra. To ensure a temporary hold, an elastic leaf (128)is arranged inside this U-shaped space and provides for a temporary holdon the bone in such a way that the blade of the hook does not riskinterfering with the marrow or other structure.

The elastic leaf (128) pushes the hook back in a posterior directionrelative to the patient and avoids lesions of the vital tissues duringthe phase of correction by rotation of the rod.

FIGS. 12 and 13 show side views of the implant, without the closurepiece.

The implant has two slots (130, 131) permitting passage of an instrumentwith two jaws which fit in the slots (130, 131), and an element whichexerts a force on the linking element (101) to effect its lateral and/orvertical displacement, so as to permit positioning of the rod in thefork by way of the closure piece (103).

FIGS. 11 through 13 show an implant whose means for anchoring in thebone are composed of a hook (126); however, the anchoring means can alsobe composed of a bone screw (140), as is shown in FIGS. 14 through 18.

FIG. 14 shows the configuration of a fixation system (100) with twoimplants/connecting assemblies (102) according to the first embodiment,and a straight linking element (101) according to the invention.

The linking element (101) is arranged in the bottom (108) of each of theconnecting assemblies (102), and the closure piece (103) closes themeans for receiving the linking element. A screw (113) firmly locks thelinking element (101) in the desired position on each connectingassembly (102).

In FIG. 14, the linking element (101) is composed of a spring (150)embedded in the support (160); however, all the embodiments of thelinking element (101) that have been discussed above are conceivable forlinking the two connecting assemblies (102).

FIGS. 15 and 16 show the configuration of a fixation system (100) withfour implants/connecting assemblies (102) according to the firstembodiment, and a single and straight linking element (101) according tothe invention.

The linking element (101) is arranged in the bottom (108) of each of theconnecting assembles (102), and the closure piece (103) closes off thereceiving means of the connecting assembly. A screw (113) firmly locksthe linking element (101) in the desired position on each connectingassembly (102).

A straight stiffening element (143) is arranged between the twoconnecting assembles situated at the center of the fixation system. Thisstiffening element (143) is also arranged in the bottom (108) of each ofthe two central connecting assemblies (102). The closure piece (103)will also lock the stiffening element (143) in the receiving means ofeach central connecting assembly, under the linking element (101), andthe screw (113) also firmly locks the stiffening element (143) in thedesired position on each central connecting assembly (102), by bearingon the linking element (101) (there is no contact between the screw(113) and the stiffening element (143)).

In FIG. 15, the linking element (101) is composed of a spring (150)embedded in the support (160); however, all the embodiments of thelinking element (101) that have been discussed above are conceivable forlinking the two connecting assemblies (102). In FIG. 16, the internalconstitution of the linking element (101) is not shown, in order not toneedlessly complicate the drawing.

FIG. 17 shows the configuration of a fixation system (100) with fourimplants/connecting assemblies (102) according to the first embodiment,and a single and curved linking element (101) according to theinvention. However, in this embodiment, the linking element (101) isonly curved between the first and second connecting assemblies andbetween the third and fourth connecting assemblies. A straightstiffening element (143) is arranged between the two connectingassemblies situated at the center of the fixation system.

The linking element (101) including the stiffening element (143) isarranged in the bottom (108) of each of the connecting assembles (102),and the closure piece (103) closes the receiving means of the connectingassembly. A screw (113) firmly locks the linking element (101) in thedesired position on each connecting assembly (102).

In FIG. 17, the linking element (101) is composed of a spring (150)embedded in the support (160); however, all the embodiments of thelinking element (101) that have been discussed above are conceivable forlinking the two connecting assemblies (102).

FIG. 18 shows the configuration of a fixation system (100) with fourimplants/connecting assemblies (102) according to the first embodiment,and a single and curved linking element (101) according to theinvention.

In this embodiment, the linking element (101) is curved between all theconnecting assemblies (102). A curved stiffening element (143′) isarranged between the two connecting assembles situated at the center ofthe fixation system.

The linking element (101) including the stiffening element (143′) isarranged in the bottom (108) of each of the connecting assemblies (102),and the closure piece (103) closes the receiving means of the connectingassembly. A screw (113) firmly locks the linking element (101) in thedesired position on each connecting assembly (102).

In FIG. 18, the linking element (101) is composed of a spring (150)embedded in the support (160); however, all the embodiments of thelinking element (101) that have been discussed above are conceivable forlinking the two connecting assemblies (102).

FIG. 19 shows the configuration of a fixation system (100) with twelveimplants/connecting assemblies (102) according to the first embodiment,each one implanted in a pedicle of a vertebra. These twelve implants arelinked to one another, in the upper part and lower part of the fixationsystem, by four straight linking elements (101) according to theinvention and, in the central part, by two rigid linking elements (145)formed by straight rods.

The connection between the rigid linking elements (145) and the linkingelements (101) according to the invention is effected with the aid offour means of connection, so-called domino blocks. Each of these meansof connection has two receiving means, for receiving a linking elementin each receiving means, and retention means formed by two nuts whichconnect fixation means to said receiving means.

As will be noted, the left-hand side shows a version of the invention inwhich the domino blocks (148) are situated at the end of the rigidlinking element (145), this element being fixed to the central pedicleswith the aid of two central implants. On the right-hand side, the dominoblocks (148) are situated at the end of the linking elements (101)according to the invention, the rigid linking element (145) being fixedto the central pedicles with the aid of four implants.

In FIG. 19, the linking elements (101) are each composed of a spring(150) embedded in the support (160); however, all the embodiments of thelinking element (101) that have been discussed above are conceivable forlinking the connecting assemblies (102) to one another or to the dominoblocks.

Linking Element According to the Invention Used with a ConnectingAssembly According to the Second Embodiment

According to the second embodiment shown in FIGS. 20 through 23, thefixation system comprises a linking element (201), an implant (202,202′) formed by a connection means and an anchoring means, the anchoringmeans being formed, for example, by a bone screw (240) having a threadof the bone type (241), as is shown in FIG. 20.

The bone-type thread (241) has a main function: that of ensuringanchoring in the bone. It may also have a secondary function: that ofreceiving the connection system for a linking element when theconnection zone (229) coincides with the upper part of the bone thread(241).

At the upper end of the screw (240), a system (205) for driving it inrotation is provided. This system, for example a hexagon, has twofunctions: on the one hand it allows the screw (240) to be driven inrotation as the latter is penetrating the bone, and, on the other hand,it also has the role of blocking rotation during final tightening of themechanism, in order to avoid excessive penetration of the screw (240)into the bone.

The connection means allows a longitudinal abutment to be formed alongthe screw (240) by means of a connector (208, 208′).

The connection means can comprise a nut (219). In this case, theinternal thread (207) of the nut (219) corresponds to the threading ofthe connection zone (229); that is to say, in the version shown in FIGS.20 through 23, to the bone thread (241) of the screw (240). The nut(219) is screwed onto the part of the screw not buried in the bone.

The nut (219) has a spherical shape (220) at its lower part. Thisspherical shape is intended to permit free positioning of the nut (219)on the connector (208) where there is a seat of the same type, or on thelinking element (201). This spherical shape (220) also serves as alongitudinal positioning abutment with the connector (208) or with thelinking element (201). In the preferred application, the connectionmeans is flanged in the connector (208) or in the linking element (201)in such a way as to secure the two parts while permitting rotation ofthe connection means on the connector (208) or on the linking element(201).

A drive system (221), for example an external hexagon, is also providedon the connection means above its spherical part (220), in such a way asto be able to adjust its height and also adjust the connector (208) orthe linking element (201) along the screw (240). As a consequence of thepossibility of rotation of the connector on the connection means, a cone(222) of entry into the connector (208) is provided for the passage ofthe ancillary for driving the connection means in rotation. By way ofexample, the angular clearance D of the axis A of the connector on theconnection means is preferably 30° when the ancillary for driving theconnection means in rotation is in place, as is illustrated in FIG. 20.

Slits (223), visible in FIG. 22, are machined in the spherical part ofthe connection means in such a way as to create a deformation upon finaltightening of the system. The aim of this deformation is to lock thescrew (226) in rotation.

The slits (223) can be positioned transversely or longitudinally.

The longitudinal slits preferably open out in the lower part of thespherical shape (220). There can be one, two, three, four, five or moreof them.

In its lower part, the connection means comprises a skirt (228), visiblein FIG. 21. This skirt (228) permits a mechanical transition between thescrew (240) and the spherical shape (220). This is because too sharp atransition would promote rupturing of the screw at the base of thespherical shape (220) in the event of dynamic forces. The skirt (228)thus permits a better distribution of the stress occurring on engagementof the screw in the nut.

This skirt (228) can be threaded at its end in order to make itspenetration into the bone easier.

The connection zone (229) of the screw (240) can be formed in the upperpart of the bone thread and can have a thread identical to the bonethread, as shown, although it can also have a different thread, or itmay not be threaded.

In the latter case, the connection means consequently has a smoothinside wall and does not constitute a nut.

In the basic version of the second embodiment, illustrated in FIG. 20,the connector (208) is provided with a single seat (224) for receiving alinking element (201). This seat (224) can be of oblong shape in thecase of a connector (208) of the closed type, or in the shape of a U,open on one of its faces, in the case of a connector of the open type.In the case of a connector (208) of the closed type, the linking element(201) must be threaded into it, whereas, in the case of an openconnector, the linking element (201) can be introduced posteriorly orlaterally on the connector.

The seat (224) for receiving the linking element (201) is provided insuch a way that the linking element (201) can bear on the sphericalshape (220) of the nut (219), as is illustrated in FIG. 20.

It will therefore be appreciated that, before final fixation, thelinking element (201) is free relative to the screw (226) in three axesof rotation and in two directions of translation:

Rotation of the connector (208) and thus of the linking element (201)about the connection means in two axes perpendicular to the screw (240);

Rotation of the connector (208) and thus of the linking element (201)about the connection means in an axis identical to that of the screw(240);

Translation of the linking element (201) in the connector (208) alongthe axis thereof;

Translation of the connector (208) and thus of the linking element (201)along the screw (240) by virtue of the possibility of adjustment of theconnection means.

In addition, the rotation of the linking element (201) about itself cangive a supplementary degree of freedom to the connecting assembly (202).

The connector (208) also has a locking system. In the basic versionillustrated in FIG. 20, this locking system is a nut (226) provided witha drive system (227) for applying a torque sufficient for goodmechanical stability of the assembly.

Such a connection piece is locked by means of the pressure of thelinking element (201) on the spherical shape (220). With the linkingelement (201) being made secure to the connector (208) by the pressureexerted by the nut (226), the degrees of freedom are then all set.

In a more complex version of the second embodiment, shown in FIGS. 21and 22, the connecting assembly (202′) can receive two linking elements(201, 201′).

The connector (208′) comprises a locking seat (225) in which one linkingelement (201) can be introduced, the other linking element (201′) beingintroduced into the seat (224).

The locking seat (225) opens out in a substantially perpendiculardirection for introduction of a plug (237), as is shown in FIG. 20.

In this version, the assembly is tightened by the force exerted on thelinking element (201′) and, at the same time, on the spherical part(220) by the linking element (201).

It is necessary to provide a plurality of connectors in order to be ableto choose the one in which the distance between the screw and thelinking element or linking elements is suitable.

FIG. 23 shows the configuration of a fixation system (200) with twoimplants/connecting assemblies (202) according to the basic version ofthe second embodiment at the ends, and with two implants/connectingassemblies (202′) according to the more complex version of the secondembodiment at the center.

A curved linking element (201) according to the invention is arrangedbetween the first and second connecting assemblies, and another curvedlinking element (201) according to the invention is arranged between thethird and fourth connecting assemblies. A curved and rigid linkingelement (245) is arranged between the two connecting assemblies situatedat the center of the fixation system.

In the connecting assemblies (202), the linking elements (201) arearranged respectively in the seats (224) of the connectors (208).

In the connecting assemblies (202′), the linking elements (201) arearranged respectively in the locking seats (225) of the connectors(208′), and the linking element (245) is arranged respectively in theseats (224) of the connectors (208′).

In FIG. 23, the curved linking elements (201) are in each case composedof a spring (250) and of a support (260), of the type shown in FIG. 6;however, all the embodiments of the linking element (201) that have beendiscussed above are conceivable for linking the connecting assemblies(202, 202′).

The invention has been described above by way of a nonlimiting example.It will be appreciated that the person skilled in the art may formdifferent configurations, especially by replacing the hook by a pediclescrew, or a vertebral screw for placement on the anterolateral face ofthe spine.

1-10. (canceled)
 11. A linking element for a spinal fixation systemdesigned to link at least two implantable connecting assemblies, whereinsaid element comprises at least partly of a support made of polymermaterial and of a rod, curved or not, substantially coaxial with saidsupport.
 12. The linking element as claimed in claim 11, wherein saidsupport has a substantially tubular or cylindrical shape.
 13. Thelinking element according to claim 11, further comprising a helicalspring having an axis substantially parallel with an axis of saidsupport and turns, said turns being at least partly embedded in saidsupport.
 14. The linking element as claimed in claim 13, wherein saidrod is substantially coaxial with said spring.
 15. The linking elementas claimed in claim 13, wherein said rod has an external diametersmaller than an internal diameter of said turns.
 16. The linking elementas claimed in claim 11, wherein said element further comprises astraight or curved stiffening element.
 17. The linking element asclaimed in claim 16, wherein said stiffening element is composed of asheet of material with a substantially U-shaped cross section.
 18. Aspinal fixation system comprising at least two implantable connectingassemblies linked by at least one linking element, said at least onelinking element comprising at least partly of a support made of polymermaterial and of a rod, curved or not, substantially coaxial with saidsupport.
 19. The spinal fixation system as claimed in claim 18, furthercomprising a stiffening element fixed at least to the two implantableconnecting assemblies.
 20. The spinal fixation system as claimed inclaim 18, further comprising at least one rigid linking element.